A significant challenge in semiconductor fabrication is to etch away a silicon oxide layer, or a portion thereof, over a nitride layer, e.g., silicon nitride, with high selectivity for the oxide layer. Since both oxide and nitride materials generally etch at about the same rate in an etch plasma, a process of providing additional selectivity must be found.
When a fluorine-substituted hydrocarbon is used as an etchant, the fluorine reacts with the carbon present to form a passivating coating of a carbon-fluorine polymer which forms over the substrate. However, this polymer is dissociated by oxygen atoms formed during the etch of the oxide layer. Thus as the silicon oxide continues to etch, the nitride layer etches at a much slower rate due to the presence of the passivating coating. However, the passivating layer is also attacked by free fluorine atoms present in the plasma, and thus the nitride also continues to be etched. Thus a selectivity over about 8:1 of silicon oxide to silicon nitride has not been achieved to date.
Since for state of the art devices having submicron dimensions, i.e., VLSI and ULSI devices, selectivity of over about 10:1. and even 30:1 and 40:1 are required, it would be highly desirable to provide an etch process for etching oxides over nitrides with high selectivities of over 10:1.